Measurement and control of exhaust gas recirculation with an oxygen pumping device

ABSTRACT

An electrochemical device and method measures the percentage of exhaust gas in the intake air/exhaust gas mixture going to the cylinders of an internal combustion engine. Two electrochemical pump cells and a support structure form a restricted volume in communication through an aperture to an ambient of intake air and exhaust gas mixture. Constant current is passed through a first pump cell to cause a portion of oxygen molecules inside the restricted volume to be pumped out. A constant voltage across the second pump cell causes substantially all the remaining O 2  molecules inside the restricted volume to be pumped out. Current flowing in the second pump cell is measured, the current being proportional to the percentage of O 2  inside the restricted volume and hence proportional to the percentage of O 2  in the intake air and exhaust gas mixture, which is a measure of % EGR.

This application is a division of application Ser. No. 07/443537, filedNov. 30, 1989, entitled MEASUREMENT AND CONTROL OF EXHAUST GASRECIRCULATION WITH AN OXYGEN PUMPING DEVICE, which is a divisionalapplication of application Ser. No. 07/222,864, filed July 22, 1988entitled: MEASUREMENT AND CONTROL OF EXHAUST GAS RECIRCULATION WITH ANOXYGEN PUMPING DEVICE, which is U.S. Pat. No. 4,909,072.

Reference is made to commonly assigned and related U.S. application Ser.No. 55,821 to Logothetis et al filed May 29, 1987 and entitled "ExhaustGas Recirculation Sensor and Method".

FIELD OF THE INVENTION

This invention relates to determining the amount of exhaust gas that isadded to the intake air going to the cylinders of an internal combustionengine.

BACKGROUND OF THE INVENTION

Exhaust gas recirculation (EGR) is commonly used in vehicles withinternal combustion engines to reduce the amount of NO_(x) produced inthe engine cylinders during combustion. Depending on engine operatingconditions, a certain amount of exhaust gas is added through an EGRvalve to the intake air that goes to the cylinders. The dilution of theintake air charge with exhaust gas results in a lower combustiontemperature and thus production of smaller amounts of NO_(x). EGR isusually measured as percentage of exhaust gas in the combined air andexhaust intake mixture. The amount of EGR is determined by the degree ofopening of the EGR valve and the difference in gas pressure across thevalve. Usually, EGR is measured with a position sensor that measures thedegree of opening of the EGR valve. This type of measurement is notreliable, however, because a) deposits can partially block the valve andb) changes in back pressure result in changes in the amount of EGR.Another method of determining EGR is based on the measurement of theflow of the exhaust gas added to the intake air. This is done bymeasuring with two pressure sensors the pressure drop across an orificethrough which the recirculated exhaust gas is passed. This method ofdetermining EGR also has problems because the orifice can be partiallyblocked by deposits.

Still another method which may be used for measuring EGR is based on themeasurement of the amount of O₂ in the combined (intake air and EGR)mixture. For an engine controlled at the stoichiometric air-to-fuelratio, the percentages of O₂ in the exhaust gas is essentially zero. Thepercentage of O₂ in the (intake air+EGR) mixture depends on the amountof EGR as shown in FIG. 1 and can be used as a measure of EGR. Theeffect of humidity in the air is indicated by showing plots for 0% and100% humidity at a temperature of 70° F. As shown, the error due tohumidity decreases as the percentage of EGR increases. Correction forthe error due to humidity is accomplished by measuring the percentage ofO₂ at zero EGR (completely closed EGR valve).

In the last 20 years, several different types of oxygen sensors based onO₂ -pumping ZrO₂ cells have been developed. Such oxygen-pumping is basedon the fact that if a current is passed through an oxygen ion-conductingelectrolyte (e.g., zirconia), oxygen is transferred (pumped) from oneside of the electrolyte to the other. Such sensors have the commoncharacteristic that their signal output is linearly proportional to theambient oxygen partial pressure. As discussed, e.g., in "HighTemperature Oxygen Sensors Based on Electrochemical Oxygen Pumping", E.M. Logothetis and R. E. Hetrick, Fundamentals and Applications ofChemical Sensors, 1986, American Chemical Society, the sensors may be ofthe single or double cell type.

In single-cell sensors, the same ZrO₂ cell is used for both, oxygenpumping and sensing. In double-cell sensors, different ZrO₂ cells areused for oxygen pumping and sensing. U.S. Pat. No. 4,547,281 to Wang isdirected to a single cell device capable of sensing the concentration ofoxygen in a volume. Double cell sensors are disclosed, e.g., in U.S.Pat. Nos. 4,272,329, 4,272,330, and 4,272,331 to Hetrick and Hetrick etal; 4,498,968 to Yamada et al; 4,645,572 to Nishizawa et al; and4,487,680 to Logothetis et al. The Hetrick, Hetrick et al and Logothetiset al patents are commonly assigned with this invention. In general, inthese two cell devices, one cell is used to pump a certain (variable)amount of O₂ out of a cavity formed between the cells and the secondcell (the sensor cell) is used to measure the reduced partial pressureof O₂ inside the cavity. As described in the patent to Logothetis et al,the structure of that device has been modified to eliminate the cavityand employs only three electrodes, instead of the common four, butoperates analogously to those of the '329, '330 and '331 patentsdiscussed above.

Embodiments of our device are similar to the two cell devices in thatthey comprise two ZrO₂ cells, which cells may define a cavity betweenthem or be similar to the Logothetis et al structure discussed above.Our device, however, does not use one cell for oxygen-pumping and thesecond for oxygen-sensing as in the two cell devices described above.Rather, our invention uses both cells as O₂ -pumping cells.

In general, measuring changes in the O₂ concentration in an intakeair/exhaust gas mixture is not trivial because the change in O₂concentration with changes in %EGR is relatively small. Of the varioustypes of oxygen sensors, the ones based on oxygen pumping are moreappropriate because they have high sensitivity to O₂, weak temperaturedependence and weak or no dependence on absolute gas pressure. In"Closed Loop Control of the EGR Rate Using the Oxygen Sensor", SAEInternational Congress and Exposition, Feb. 29-Mar. 4, 1988, TechnicalPaper No. 880133, M. Hishida, N. Inoue, H. Suzuki, and S. Kumargaidisclose a device comprising an oxygen pump cell and a sensor celluseful to measure %EGR. For measuring small changes in O₂ at high O₂concentrations, however, an O₂ sensor with higher sensitivity isdesirable. The present invention describes a method for measuring EGRand an EGR sensor which overcomes this problem.

SUMMARY OF THE INVENTION

This invention provides a method for measuring the percentage of EGR bymeasuring the percentage of O₂ in the intake air exhaust gas mixture(herein taken to mean "the mixture of intake air and exhaust gas") withan electrochemical device, which O₂ concentration is proportional to thepercentage of EGR. The method comprises restricting communicationbetween an ambient of an intake air and exhaust gas mixture and arestricted volume and passing a constant current through a firstelectrochemical pump cell so that an electrode, of the first pump cell,inside the restricted volume is biased negatively causing a portion ofthe oxygen molecules inside the volume to be pumped out by the currentflowing through the first pump cell. The method further comprisesapplying a constant voltage across a second electrochemical pump cell sothat an electrode, of the second pump cell, inside the restricted volumeis biased negatively, the constant voltage across the second pump cellbeing sufficient to cause substantially all of the remainder of theoxygen molecules inside the restricted volume to be pumped out by acurrent flowing through the second pump cell but less than that capableof disassociating CO₂ or H₂ O molecules. The method also comprisesmeasuring a current flowing through the second pump cell, the secondpump cell current being proportional to the percentage of O₂ moleculesinside the restricted volume not pumped out by the first pump cell andalso proportional to the percentage of O₂ molecules in the intake airand exhaust gas mixture. This invention is particularly advantageous foruse in measuring the O₂ concentration in high-O₂ mixtures. Correctionfor variable humidity in air is accomplished by measuring the percentageof O₂ when the EGR valve is closed (0% EGR).

According to another aspect of this invention, an electrochemical devicefor measuring EGR includes a first solid electrochemical pump cellhaving a first pair of opposed electrodes and a second solidelectrochemical pump cell having a second pair of opposed electrodes. Asupporting structure is coupled to the first and second pump cells toform with them a restricted volume. This volume communicates with theambient atmosphere (the intake air and exhaust gas mixture when used inthe method of the invention) through an aperture or collection ofapertures. In operation according to the method of the invention, aconstant current is passed through the first pump cell (by a firstexternal circuit means coupled to the first pump cell) so that a portion(volume %) of the oxygen molecules inside the restricted volume ispumped out. In operation, a constant voltage is applied across thesecond pump cell (by a second external circuit means coupled to thesecond pump cell) of sufficient magnitude to cause substantially all ofthe remainder of the O₂ molecules present inside the restricted volumeto be pumped out and achieve a saturation current but less than thatwhich will cause decomposition of any gas molecules containing oxygen,e.g., CO₂ or H₂ O, that may be present in the restricted volume. Thefirst external circuit means is adapted to apply an external voltageacross the second pump cell which is generally less than about 0.8volts, more preferably between about 0.2 and 0.8 volts. The saturationcurrent of the second pump cell is proportional to the percentage of O₂inside the volume, not pumped out by the first pump cell proportional tothe percentage of O₂ in the intake air/exhaust gas mixture and to thepercentage of EGR therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph relating the percentage of O₂ in a combined intake airand exhaust mixture to the percentage of EGR.

FIG. 2 is a schematic of an electrochemical device for measuring EGRaccording to one embodiment of this invention.

FIG. 3A is a schematic of a single-cell oxygen pumping device accordingto the prior art.

FIG. 3B is a graph relating the current voltage characteristics of thedevice of FIG. 3A.

FIG. 4 is a graph relating the percentage of O₂ in the volume of thedevice of FIG. 2 as a function of the percentage of EGR.

FIG. 5 is a schematic of an electrochemical device for measuring the EGRaccording to a second embodiment of the present invention.

FIGS. 6A and 6B are schematics of electrochemical devices for measuringEGR in accordance with embodiments of the present invention.

FIG. 7 is a schematic of an electrochemical device having a planarconfiguration for measuring EGR in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention teaches a method for determining EGR based on themeasurement of the amount of O₂ in the intake air and EGR mixture byemploying an electrochemical oxygen pumping device.

As mentioned above, several types of oxygen sensors based on oxygenpumping with ZrO₂ electrochemical cells exist. These sensors haveseveral characteristics that make them suitable for lean A/F ratios(i.e., high EGR). However for measuring small changes in O₂ at high O₂concentration, see FIG. 1, an O₂ sensor with higher sensitivity isdesirable.

This invention measures EGR by measuring the percentage of O₂ in the airplus exhaust gas mixture with a sensor such as that shown in FIG. 2.This sensor 20 has two electrochemical cells 21 and 31 arranged so thata restricted volume 40 is defined. Volume 40 is linked to the ambientatmosphere (air plus EGR) through an aperture 41 or a collection ofapertures. Each of the two cells consists of a platelet 22, 32 made froman oxygen conducting solid electrolyte such as ZrO₂, and two electrodes23, 24; 33, 34 applied on the two sides of the platelets. Theseelectrodes are made from platinum or some other material according toprocedures well established in the area of oxygen sensors.Electrochemical cells 21 and 31 are operated as oxygen pumps by passingcurrents I₁ and I₂ through them. Advantageously, a heater 29 ispositioned adjacent sensor 20 to provide an elevated temperature ofabout at least 500° C. suitable for operation of sensor 20.

In order to understand the operation of device 20, first consider singleZrO₂ cell device 60 of FIG. 3A which is useful as a lean O₂ -sensor. Ithas a single oxygen pumping cell 61 made from a ZrO₂ platelet with twoplatinum electrodes 63 and 64 arranged in a structure so that a volume65 is defined. Volume 65 communicates with the ambient gas through anaperture 66. When a voltage is applied across cell 61 so that electrode63 is negative, a current I₃ passes through the ZrO₂ material as aresult of motion of oxygen ions from electrode 63 to electrode 64.

As the oxygen ions formed at electrode 63 travel through the ZrO₂platelet to electrode 64, more O₂ molecules from the gas phasedissociate and react with electrons at electrode 63 to form oxygen ions(O⁼). By means of this electrochemical reaction, as oxygen ions aredepleted at electrode 63 (in traveling to electrode 64) more oxygen ionsare formed from O₂ gas molecules in volume 65. By means of an inverseelectrochemical reaction, oxygen ions at electrode 64 are released as O₂molecules into the ambient gas. The net effect of the current throughthe cell is to pump O₂ out of volume 65. Because of the lowerconcentration of O₂ inside volume 65, there will be a diffusional fluxof O₂ from the ambient (intake air plus exhaust gas mixture) into volume65 through aperture 66. Under steady state conditions, the diffusionalflux of O₂ into volume 65 will be equal to the flux of O₂ Pumped out ofvolume 65 by the pumping current.

If the single cell device 60 is used to measure the percentage of O₂ inan exhaust gas/intake air mixture, for small voltages (0.2-0.8 V) thecurrent increases with voltage as more oxygen is pumped out of volume 65and reaches a saturation current, I_(s). This corresponds to thecondition that all oxygen inside volume 65 is pumped out by the current.The saturation current I_(s) is proportional to the percentage of O₂ inthe ambient. The voltage across the pump cell is maintained below about0.8 volts so as to not decompose any gas molecules containing O₂, suchas CO₂, present in volume 65. The current voltage (I-V) characteristicsof single cell prior art device 60 (for various percentages of O₂) underthese conditions is shown in FIG. 3B.

The relationship between the percentage of O₂ in intake air/exhaust gasmixtures and the percentage of EGR therein is shown in FIG. 1. FIG. 4shows the same relationship except at lower percentages of O₂ inside thevolume. Sensitivity is generally defined as the percentage change in Yfor a given change in X, as relating components from a conventional X-Yaxes graph. It can be seen from FIG. 1 that for the same change in %EGR(X axis) at concentrations higher and at lower EGR (FIG. 4), there is alarger percentage change in O₂ (Y axis) at the higher EGR's (lower O₂concentration). Thus the prior art single cell device is more sensitiveto changes in EGR at low oxygen concentrations (high EGR) than it is athigh oxygen concentrations (low EGR). Thus while device 61 of FIG. 3Acan be suitably used for the measurement of EGR in a low-O₂ environment,it is not optimally sensitive in high O₂ (i.e. low EGR) environments.

The measurement of EGR in a high-O₂ intake air/exhaust gas mixture canbe accomplished, however, with a device according to the Presentinvention. Referring to FIG. 2, device 20 can be used to measure thepercentage of O₂ in such a gas mixture. The device comprises a first anda second oxygen pump cell. The first oxygen pumping cell pumps out aportion (e.g., 10 volume %) of the oxygen molecules from volume 40 so asto reduce the percentage of O₂ molecules in the gas mixture in thevolume. This, in effect, modifies the percentage of O₂ molecules in thehigh O₂ (20-10%) intake air/exhaust gas mixture of FIG. 1 to the lowerO₂ (10-0%) mixture of that of FIG. 4. The portion of oxygen molecules ispumped out of volume 40 by passing a constant current through cell 21 sothat electrode 24 is negative. The current passed through cell 21 isthat which is sufficient to cause cell 21 to pump the desired amount(portion) of the O₂ molecules out of volume 40. Preferably, the amountof O₂ pumped out of volume 40 by the first oxygen pump cell 21 equalsthe percentage of O₂ (by volume) present in the intake air multiplied by(100 minus at the maximum percentage of EGR employed by that particularinternal combustion engine). Thus, removing 10 volume % of the O₂ as inthe embodiment above would be optimal for an engine in which the maximumamount of EGR generally employed in the engine gas mixture is 50% byvolume. In an engine in which the maximum amount of EGR generallyemployed in the engine gas mixture is 30% by volume, 14 volume % (i.e.70 multiplied by 20%)cell 21 of the device would optimally remove of theO₂ molecules, leaving about 6 volume % (i.e., 30 times 20%) of the O₂molecules in volume 40 at zero EGR. The current necessary to remove thedesired portion of O₂ molecules from the volume would depend on suchfactors as the type and dimensions of aperture 41, and temperature.Selection of the optimum current necessary to pump the desired portionof the O₂ molecules out of volume 40 by means of cell 21 will beapparent to those skilled in the art in view of the present disclosure.

According to the embodiment of the invention device of FIG. 2, thepercentage of O₂ in the reduced oxygen mixture inside volume 40 ismeasured by the second oxygen pump cell 31. A constant voltage of lessthan 0.8 volts, generally between 0.2 and 0.8 volts, is applied acrossthe second oxygen pump cell 31 of the invention device of FIG. 2, withelectrode 33 being negatively biased, to pump out substantially theremainder of O₂ molecules inside volume 40. The voltage across the pumpcell is maintained below about 0.8 volts so as to not decompose any gasmolecules containing O₂, such as CO₂, present in volume 40. Forsufficiently large voltages, about 0.5 to 0.8 volts, a saturationcurrent is achieved in cell 31. An external circuit means coupled topump cell 31 is used to measure the saturation current flowing throughthe second pump cell 31. This saturation current, I_(s), is a measure ofthe percentage of O₂ molecules in the volume 40 not pumped out by firstpump cell 21 and also is proportional to the percentage of O₂ moleculesin the intake air and exhaust gas mixture.

In effect, the reduced-O₂ mixture inside volume 40 (effected by thepumping out of a portion of the O₂ molecules from volume 40 by cell 21)simulates a low-O₂ mixture. This allows the device to measure thepercentage of O₂ in a high-O₂ mixture with the sensitivity obtained whenmeasuring low-O₂ mixtures. Thus the device of this invention provides amore sensitive measurement of EGR in a high-O₂ intake air/exhaust gasmixture as compared to that obtained by the single O₂ pumping prior artdevice 3A.

The device of FIG. 2 operates under the assumption that cell 21 can pumpthe desired portion of oxygen entering volume 40 through aperture 41 sothat only the remainder of the O₂ reaches cell 31. If this is not thecase with the device structure of FIG. 2, the desired condition can beaccomplished by modifying its structure to that of device 50 as shown inFIG. 5. In this structure, a porous layer 38 is deposited on top of theinner electrode of cell 31. This porous layer is made from ZrO₂ or aninert material (e.g. spinel or aluminum oxide) and acts as a hindrance(barrier) to O₂ diffusion so that the desired portion of O₂ is pumpedout by cell 21.

Several other device configurations are possible. According to otherembodiments of this invention, the two pumping cells are more stronglydecoupled by placing between them barriers to oxygen diffusion. Forexample, the porous layer in the device of FIG. 5 may be replaced with a"wall" 42 having an aperture 43 as shown in device 50A of FIG. 6A.Another type of configuration is shown in device 50B of FIG. 6B. Device50B uses a pump cell to remove a portion of the O₂ and a pumpcell/sensor cell structure 70 (similar to the sensor structure describedin U.S. Pat. No. 4,272,329 by Hetrick et al) to remove and measure theremaining O₂.

The electrochemical device for the measurement of EGR disclosed here canalso be made in a planar configuration. FIG. 7 shows one embodiment of aplanar device 80 according to this invention. One starts with a denseZrO₂ platelet 82 and deposits porous platinum electrodes 83 and 84 onboth sides of platelet 82 to form pump cell 81. A porous layer 89 madeof ZrO₂ or an inert material (e.g. spinel or aluminum oxide) isdeposited on top of platinum electrode 83 to form a barrier to diffusionof O₂ molecules. A porous platinum electrode 85 is deposited on layer 89followed by another porous layer 86 of ZrO₂ Finally, a porous platinumelectrode 87 is deposited on top of layer 86. Porous ZrO₂ layer 86 andplatinum electrodes 85 and 87 form pump cell 88. As in the case of thedevice of FIG. 2, a constant current is passed through pump cell 88 topump a portion of the oxygen out of the porous parts of the structure,and a constant voltage of between about 0.2 and 0.8 volts is appliedacross pump cell 81 to pump out substantially all of the remaining O₂inside the porous layer 89. The saturation current of pump cell 81 isproportional to the percentage of EGR in the gas mixture.

Various modifications and variations will no doubt occur to thoseskilled in the art to which this invention pertains. For example, theparticular construction of the two cell oxygen pumping device may bevaried from that disclosed herein. These and all other variations whichbasically rely on the teachings through which this invention hasadvanced the art are properly considered within the scope of thisinvention.

We claim:
 1. An electrochemical device for measuring the percentage ofexhaust gas recirculation (EGR) in an intake air and exhaust gas mixtureof an internal combustion engine, said electrochemical deviceincluding:a generally planar first electrochemical pump cell including arelatively dense ZrO₂ platelet with porous platinum electrodes on twosides of said platelet; a first porous ZrO₂ layer deposited on oneelectrode of said first pump cell; a generally planar secondelectrochemical pump cell including a first porous platinum electrode, asecond porous ZrO₂ layer and a second porous platinum electrodedeposited successively on said first porous ZrO₂ layer; a second cellexternal circuit means coupled to said second pump cell for passing aconstant current through said second pump cell so that said secondelectrode is biased positively causing a portion of the oxygen moleculesinside said first porous ZrO₂ layer to be pumped out by said currentflowing through said second pump cell; a first cell external circuitmeans coupled to said first pump cell for applying a constant voltageacross said first pump cell so that the exposed electrode is biasedpositively, said constant voltage across said first pump cell beingsufficient to cause substantially all of the remainder of the oxygenmolecules inside said first porous ZrO₂ layer to be pumped out by acurrent flowing through said first porous ZrO₂ layer but less than thatcapable of disassociating CO₂ or H₂ O molecules; and a third externalcircuit means coupled to said first pump cell for measuring the currentflowing through said first pump cell, said current being proportional tothe percentage of O₂ inside said first porous ZrO₂ layer not pumped outby said second pump cell and also proportional to the percentage of O₂in said intake air and exhaust gas mixture.
 2. The electrochemicaldevice as recited in claim 1, further comprising a heater to maintainthe temperature of said device at least about 500° C.
 3. Theelectrochemical device as recited in claim 1, wherein the percentage ofoxygen molecules pumped out of said first porous ZrO₂ layer by saidsecond pump cell equals the percentage of oxygen present in said intakeair and exhaust gas mixture of said engine at maximum exhaust gasrecirculation.
 4. The electrochemical device as recited in claim 1,wherein said first external circuit means is adapted to apply acrosssaid first pump cell a constant voltage in the range 0.2 to 0.8 volts.5. A method for measuring the percentage of exhaust gas recirculation inan intake air and exhaust gas mixture of an internal combustion engineincluding the steps of:forming a generally planar first electrochemicalpump cell including a relatively dense ZrO₂ platelet with porousplatinum electrodes on two sides of said platelet; depositing a firstporous ZrO₂ layer on top of one of the electrodes of said pump cell;forming a generally planar second electrochemical pump cell including aporous platinum electrode, a second porous ZrO₂ layer and a porousplatinum electrode deposited successively on top of said first porousZrO₂ layer; passing a constant current through said second pump cell sothat the exposed electrode is biased positively causing a portion ofoxygen molecules inside said first porous ZrO₂ layer to be pumped out bythe current flowing through said second pump cell; applying a constantvoltage across said first pump cell so that the exposed electrode isbiased positively, said constant voltage across said first pump cellbeing sufficient to cause substantially all of the remainder of theoxygen molecules inside said first porous ZrO₂ layer to be pumped out bya current flowing through said first porous ZrO₂ layer but less thanthat capable of disassociating CO₂ or H₂ O molecules; and measuring thecurrent flowing through said first pump cell, said current beingproportional to the percentage of O₂ inside said first porous ZrO₂ layernot pumped out by second pump cell and also proportional to thepercentage of O₂ in said combined intake air and exhaust gas mixture. 6.A method for measuring exhaust gas recirculation as recited in claim 5,wherein the variability of the humidity in the air is corrected byadditionally measuring the percentage of O₂ in the intake air andexhaust gas mixture when an exhaust gas recirculation valve is closedthereby stopping exhaust gas recirculation.
 7. The method for measuringexhaust gas recirculation as recited in claim 5, wherein the percentageof oxygen pumped out of said first porous ZrO₂ layer equals thepercentage of oxygen present in said intake air and exhaust gas mixtureof said engine at maximum exhaust gas recirculation.
 8. The method formeasuring exhaust gas recirculation as recited in claim 5, wherein saidconstant voltage applied across said first pump cell is in the range 0.2to 0.8 volts.